Step One: Better Audio Response with the FT8900 Yaesu MH-48A6J By Improving Acoustics
8900


I picked up an FT8900 for my home and another for my vehicle and have been generally pleased with them. But right from the start, I got reports that both of them were quite weak in the transmit audio department. If you own a Yaesu FT8900 and use the MH-48A6J microphone, you also may have received a few reports that the transmit audio is fairly low and has a pronounced roll-off of low frequencies resulting in a fairly thin sound when compared to some other radios. While this low frequency attenuation might be desirable in certain applications, it really is tough to live with if you prefer your audio to be more natural sounding with better low end frequency response.  Surprisingly, it appears that Yaesu went out of its way to mechanically block the port in the front of the mic case to help produce the restricted audio effect.  Taking the mic apart reveals that only 2 of the 3 tiny mic element holes in the front of the case are actually open, and they are quite small, forming an audio high pass port. They also saw fit to place the mic element in a small rubber boot that blocks much of the front of the electret mic cartridge, which may also reduce the frequency response of the element. I chose to work on these mechanical restrictions to see what the result might be, and found some improvement since opening the mic up.  Directions for the mechanical modifications of the mic are below:

  1. Remove the back of the microphone case. (3 screws)

  2. Lift off the PTT lever.

  3. Lift off the UP/DOWN key assembly at the top of the mic.

  4. Remove the 3 visible screws, one in the upper left, two near the bottom end of the mic.

  5. Remove the small screw fastening the PTT micro switch through the board to the mic front casing.

  6. Gently lift the board away, taking care to not stress the wires to the electret element.

  7. Gently work a small screw driver around the mic element rubber boot, and slowly work it out from the mic front casing. Be gentle and take your time. I found that there was a spot near the top of the mic housing where I could work the screwdriver in to get things started. Lift the mic element out of its housing when you have it loosened up.

  8. Remove the plastic baffle insert in the molded mic housing, it should fall out easily (see picture below).

  9. Using a 1/16 inch drill bit, drill out the two existing holes in the front of the microphone and the third fake one. Then slide the drill bit back and forth to transform the three holes into a slot. (See the picture below for an idea of how this will look when you are done.)

10. Carefully remove the black rubber boot from the electret element, again, take your time because it is fairly tight.

11. Enlarge the hole through the front of the boot to open access to more of the front of the mic element.

12. Place the black rubber boot back onto the electret element.

13. Wad up a tiny bit of cotton and place it inside the mic housing where the baffle was. Use enough to loosely cover the bottom of the little chamber that houses the electret. This helps to act as a pop filter. Using too much may result in a muffled sound, you really just need a few wisps.

14. Carefully guide the mic element back into the molded mic housing. Make sure it is seated all way down into the cylinder.

15. Mount the circuit board inside the front of the microphone and tighten down the screws. Then wad some cotton to fill the mic housing. This microphone seems to have some odd acoustic resonances from the hollow space in the back of the microphone. The cotton will reduce this effect greatly.
Note: Remove the plastic baffle that the arrow is pointing to.
baffle position

Note: I got a C+ in shop but the slot still came out fine!
MH48 Slot
Step Two: Better Audio Response with the FT8900 Yaesu MH-48A6J By Increasing Coupling

The worst culprit regarding restriction of the low end audio appears to be in the speech amplifier built into the remote display head of the radio. A circuit diagram for the PANEL-SUB-UNIT is located on the back of the MAIN UNIT block diagram. One glance will reveal that they have coupled the output of the mic through a .0056 uf capacitor to the input of the first stage of the speech amp. A larger value would have produced much smoother response, and much more audio. However, this circuit board is populated with devices that are the size of a flea, and I would not even attempt to mess with it! All is not lost though, you can still bring about some improvement right in the microphone unit itself. If you examine the schematic below, and trace the audio path from the output of the electret mic element to pin 5 of the connection plug for the mic cord, you will see why the low end response of this microphone is so restricted. The 0.1uf capacitor (C5003) in parallel with the 1k resistor forms a high pass circuit that effectively blocks the passage of a good percentage of the normal low end voice frequencies. The easiest way around this is to simply bridge a cap from the mic output to pin 5. All the RF bypassing and decoupling will still work effectively, but the audio path will now bypass the 0.1uf (C5003) frequency shaping capacitor and the 0.33uf (C5002) coupling cap. I chose a tantalum 10uf cap (RS part number 272-1436) for this task. This will boost the low end frequency response and also raise the total audio output of the mic slightly, which in my opinion is a good thing because the FT8900 seems a little lean in that department with the stock setup. Directions for completing this modification are below.
schematic


  1.  Remove the back of the microphone case.  (3 screws)

  2.  Lift off the PTT lever.

  3.  Lift off the UP/DOWN key assembly at the top of the mic.

  4.  Locate the caps that are to be bridged. (Note in the picture below the caps with white arrows pointing to them.)
caps

  5.  Examine the picture below to see where the two solder connection points are. It is important to pay attention to the polarity of the cap as you are connecting to the voltage feedpoint of the mic element. In the picture below, the plus lead of the cap is soldered to the left side of the cap labeled V104 (C5003) and the other leg is soldered to the right side of the cap labeled V334 (C5002). I placed a chunk of double sided foam tape behind the cap to help hold it steady when soldering and to prevent the cap from vibrating or stressing the solder points after the mic is put back into service. After bending the leads of the cap to fit, tin the leads with a low wattage iron using a very sharp tip. Then press it into place. When the cap is in position, solder it to the pads at the ends of the surface mount devices. You won't need much solder, and don't linger on those tiny solder pads for too long!
cap

 6.  After confirming that the cap is securely soldered, reassemble the mic and you are done with this part of the modification.
Step Three: More Audio by Increasing the Deviation of the FT8900

The final step to raising the audio level of the FT8900 involves accessing the service menu and increasing the deviation setting. I would urge you to record your factory settings before you adjust them, should you wish to set the rig back to its stock setting for some reason. I ended up setting all bands to the maximum setting in the service menu, and this resulted in full 5 kc deviation on all bands except for 10m, which hits the wall at approximately 3.1 kc. The procedure for deviation adjustment is below:

FT-8900 Entering the Alignment Mode:

A.  Press and hold in the “Left” band (V/M) key and the Hyper Memory #6 key while turning the radio
      on. Once the radio is powered up, release these two keys.

B.  Press the following front panel keys in sequence from left to right:

      Left Side  [LOW]   [V/M]   [HM]   [SCN]   Right Side   [LOW]   [V/M]   [HM]   [SCN]

C.  You will now see “A-0 REF .xxH” on the display. This signifies that the transceiver is now in the
      “Alignment” mode.

FT-8900 TX Deviation Adjustment (A-4 DEV): Note that user menu item 43 should be set to WIDE.

A.   Press the “Sub” band DIAL knob momentarily, if needed, to switch the “Main” band to the “Right”  
       side.  If “Main” already appears on the “Right” side, then proceed to the next step.

B.   Tune the “Right” band frequency to 440.050 MHz, then set the Transmit Power Level to “LOW”.

C.   Press and hold in the “Left” DIAL knob momentarily to step through the A-0, A-1, A-2, A-3
       alignment menus until you arrive at the alignment parameter that reads “A-4 DEV .xxH.”

D.   Inject a 1 kHz audio tone at a level of 50 mV from an Audio Generator. If you don't have an audio generator, read on.

E.   Press the PTT switch to activate the transmitter and adjust the “Left” DIAL knob, as needed, so that  
       the deviation meter reading is 4.5 kHz. If no Audio Generator is available adjust the deviation setting to (DEV.3FH).

F.   Tune the “Right” band frequency to 146.050 MHz, then set the Transmit Power Level to “LOW”.

G.   Press the PTT switch to activate the transmitter, and adjust the “Left” DIAL knob, as needed, so that
       the deviation meter reading is 4.5 kHz. Again, if no Audio Generator is available adjust the deviation setting to (DEV.3FH).

H.   Press the “Left” DIAL knob momentarily to switch the “Main” band to the “Left” side.

I.    Tune the “Left” band frequency to 52.050 MHz, then set the Transmit Power Level to “LOW”.

J.    Press the PTT switch to activate the transmitter, and adjust the “Right” DIAL knob as needed, so that
      the deviation meter reading is 4.5 kHz. Again, if no Audio Generator is available adjust the deviation setting to (DEV.3FH).

K.  Tune the “Left” band frequency to 29.050 MHz, then set the Transmit Power Level to “LOW”.

L.   Press the PTT switch to activate the transmitter, and adjust the “Right” DIAL knob to the maximum
      value available (DEV.3FH). This will increase your deviation on 10 meters to just a bit over 3 kHz.

M.  Turn off the FT-8900 and then power on as you would normally, to operate as usual.
Summary of the Audio Modifications

The end result of the acoustic improvement, heavier coupling capacitance, and deviation adjustments, should be a much fuller sounding transmit signal. I have been very pleased with the audio reports I have received since completing the changes. Reports indicate that the FT8900 sounds very natural, with a good balance of highs and lows throughout the voice frequencies. Best of all, it is simply much louder and easier for other stations to hear. Audio clips in mp3 format of my FT8900 and MH48 before and after modification are below, along with an Adobe Audition screenshot of each of their waveforms.
MH48 Stock Audio (Receiver: ICOM 756 Pro III on 52.525 mhz captured with Adobe Audition)

1
Stock MH48 microphone with factory deviation setting of DEV.2DH



MH48 Modified Audio (Receiver: ICOM 756 Pro III on 52.525 mhz captured with Adobe Audition)

2
Modified MH48 microphone with front port slotted, 10uf coupling cap, and deviation set to DEV.3FH

The total time for modifying each mic and adjusting each of my FT8900 rigs was about 45 minutes, but then again, I have never been accused of rushing anything, and my experience with surface mount construction is very limited, so chances are you will get it done faster.


Disclaimer:  This modification is offered in the spirit of Ham Radio experimentation. If you end up damaging or destroying your MH-48A6J or any other equipment, I am not responsible.
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